Typically, silicon is used as a basic material for solar cells. The starting point in the manufacture of a silicon solar cell is a crystalline silicon wafer in which a p-n junction is formed by means of doping. The procedure thereby is usually such that a p-doped silicon base material is used and an n-doped emitter layer is formed in order to configure the p-n junction. Both terminals of the p-n junction are then typically contacted via the frontside and backside of the silicon wafer.
Within the framework of backside contacting, what is referred to as a “back-surface field” is configured on the silicon wafer backside in order to prevent recombination losses. For this purpose, an aluminum layer is typically deposited on the silicon wafer backside, covering the entire surface, the aluminum layer then being subjected to a sintering process in order to superficially fuse the aluminum layer at the boundary surface with the silicon wafer and to alloy the aluminum into the silicon wafer. During operation of the solar cell, the resulting highly p-doped silicon surface layer acts as a back-surface field which drives the minority current carriers which reach the backside by means of diffusion back into the silicon wafer, thus preventing recombination losses.
In order to save costs, the introduction of aluminum into the silicon wafer is preferably carried out by means of a screen-printing process in which an aluminum paste is deposited on the silicon wafer backside and cured into the silicon wafer at 700 to 900° C. after drying in order to configure the back-surface field.
The problem with the screen-printed sintered aluminum layer on the silicon wafer backside, however, is that it is only to a very limited extent suitable for soldering contact connectors in order to connect the solar cell, particularly solder wires. In order to achieve a technically useable capacity in solar cells, a plurality of solar cells is usually connected in series to form a solar module. In the series connection of the solar cells, the frontside contacts of a solar cell are soldered to the backside contacts of the next solar cell by means of the contact connectors, i.e. typically solder wires, in order to form what is known as a string of cells.
Such a soldering connection between the screen-printed sintered aluminum layer and the contact connector is, however, extremely unstable, predominantly due to the high probability of cohesive failure of the aluminum layer. For contacting the silicon wafer backside, aluminum-free contact regions are usually provided on the silicon wafer backside. These contact regions comprising a width of several millimeters are provided with a silver-containing screen-printed metallization layer to which the contact connectors may be soldered. The silver screen-print is in this context usually carried out prior to the aluminum screen-print, with the two screen-print surfaces overlapping slightly.
However, since silver is a relatively expensive resource, it would be desirable for such additional silver contacts to become redundant. As an alternative for backside contacts made of silver, EP 2 003 699 A2 suggests to deposit a screen-printed aluminum layer over the entire surface of the silicon wafer backside and to subsequently cure it in order to form a back-surface field at the boundary surface. Subsequently, a further layer of material suitable for soldering, preferably tin, is deposited over the entire surface or in partial regions of the screen-printed sintered aluminum layer. This second layer is alloyed into the aluminum layer beneath in a point-wise manner by means of acoustic-mechanical irradiation, superficial fusing or thermal spraying, resulting in conductive contact regions to which contact connectors, particularly solder wires, may be soldered. Ultrasonic loading as well as superficial fusing or, respectively, thermal spraying, however, confers high kinetic energy to the solder particles to be alloyed, which may result in damage to the silicon wafer surface and thus in a deterioration of electrical properties of the solar cell. For alloying the material suitable for soldering into the screen-printed sintered aluminum layer, only a very small process latitude is thus available which is to be precisely observed in order to prevent the danger of negatively influencing the electrical solar cell parameters.